Growth Biorhythms of the European Pearl Mussel Margaritifera Margaritifera (bivalvia, Margaritiferidae) of the Varzuga River Population (murmansk Oblast)

The individual linear growth of 90 specimens of the freshwater pearl mussel Margaritifera margaritifera was studied by measuring the successive annual rings on the surface of the shell. It was found that the growth of each mollusk can be described using a united equation, a particular case of which is the Bertalanffy equation with an average growth retardation coefficient a = 0.048 year –1 and the coefficient of the initial conditions d = 5.3 mm/year. It was noted that the growth of mollusks is accompanied by three regular biorhythms with periods of 13.4, 6.8, and 4.0 years, with the first two biorhythms fading, the latter having a constant amplitude. It was found that a decrease in the amplitude of fading biorhythms can be described by a power equation with the same coefficients. Questions related to the nature of the biorhythms identified are discussed.